The p53 tumor suppressor exerts anti-proliferative effects, including growth arrest, apoptosis, and cell senescence, in response to various types of stress (Levine, 1997; Prives and Hall, 1999; Vogelstein et al., 2000). Mutations within the p53 gene have been well documented in more than half of all human tumors (Hollstein et al., 1994). Accumulating evidence further indicates that, in the cells that retain wild-type p53, other defects in the p53 pathway also play an important role in tumorigenesis (Prives and Hall, 1999; Lohrum and Vousden, 1999). The molecular function of p53 that is required for tumor suppression involves its ability to act as a transcriptional factor in regulating downstream target gene expression (reviewed in Nakano and Vousden, 2001; Yu et al., 2001).
p53 is a short-lived protein whose activity is maintained at low levels in normal cells. Tight regulation of p53 is essential for its effect on tumorigenesis as well as maintaining normal cell growth. The precise mechanism by which p53 is activated by cellular stress is not completely understood; it is generally thought to involve mainly post-translational modifications of p53, including phosphorylation and acetylation (reviewed in Appella and Anderson, 2000). Early studies demonstrated that CBP/p300, a histone acetyl-transferase (HAT), acts as a coactivator of p53 and potentiates its transcriptional activity as well as biological function in vivo (Gu et al., 1997; Lill et al., 1997; Avantaggiati et al., 1997). Significantly, the observation of functional synergism between p53 and CBP/p300 together with its intrinsic HAT activity led to the discovery of a novel FAT (transcriptional factor acetyl-transferase) activity of CBP/p300 on p53. This finding also indicates that acetylation may represent a general functional modification for non-histone proteins in vivo (Gu and Roeder, 1997).
Through the use of site-specific acetylated p53 antibodies, CBP/p300 mediated acetylation of p53 was further confirmed in vivo by a number of studies (Sakaguchi et al., 1998; Liu et al., 1999; Luo et al., 2000; Kobet et al., 2000; Ito et al., 2001). Significantly, the steady-state levels of acetylated p53 are stimulated in response to various types of stress, indicating the important role of p53 acetylation in stress response (reviewed in Ito et al., 2001).
By introducing a transcriptionally defective p53 mutant (p53Q25S26) into mice, it was found that the mutant mouse thymocytes and ES cells failed in undergoing DNA damage-induced apoptosis (Chao et al., 2000; Jimenez et al., 2000). Interestingly, this mutant protein was phosphorylated normally at the N-terminus in response to DNA damage but could not be acetylated at the C-terminus (Chao et al., 2000), supporting a critical role of p53 acetylation in p53-dependent apoptotic response (Chao et al., 2000; Luo et al., 2000).
Furthermore, it has been found that oncogenic Ras as well as PML can upregulate the levels of acetylated p53 in normal primary fibroblasts, and also induce premature senescence in a p53-dependent manner (Pearson et al., 2000, Ferbeyre et al., 2000). p53 acetylation may also play a critical role in protein stabilization (Rodriguez et al., 2000; Nakamura et al., 2000; Ito et al., 2001). In addition, another independent study showed that acetylation, but not phosphorylation of the p53 C-terminus, may be required to induce metaphase chromosome fragility in the cell (Yu et al., 2000).
In contrast, much less is known about the role of deacetylation in modulating p53 function. The acetylation level of p53 is enhanced when the cells are treated with histone deacetylase (HDAC) inhibitors such as Trichostatin A (TSA). This observation led to the identification of a HDAC1 complex which is directly involved in p53 deacetylation and functional regulation (Luo et al., 2000; Juan et al., 2000).
PID/MTA2, a metastasis-associated protein 2, acts as an adaptor protein to enhance HDAC1-mediated deacetylation of p53, but this activity can be completely repressed by TSA (Luo et al., 2000). In addition, Mdm2, a negative regulator of p53, actively suppresses CBP/p300-mediated p53 acetylation, and this inhibitory effect can be abrogated by tumor suppressor p19ARF, suggesting that regulation of acetylation also plays a critical role in the p53-MDM2-p19ARF feed back loop (Ito et al., 2001; Kobet et al., 2000).
The yeast silent information regulator 2 (Sir2) protein belongs to a novel family of histone deacetylases (reviewed in Guarente, 2000; Shore, 2000). Sir2 activity is nicotinamide adenine dinucleotide (NAD)-dependent, but can not be inhibited by TSA (Imai et al., 2000; Landry et al., 2000a; Smith et al., 2000). The NAD-dependent deacetylase activity of Sir2 is essential for its functions, and this activity also connects its biological role with cellular metabolism in yeast (Guarente, 2000; Imai et al., 2000; Lin et al., 2000; Smith et al., 2000). Recently, mammalian Sir2 homologs have been found to also contain the NAD-dependent histone deacetylase activity (Imai et al., 2000; Smith et al., 2000), further supporting the notion that the enzymatic activity is key to elucidating the molecular mechanism for its mediated functions (Min et al., 2001; Finnin et al., 2001).
Among Sir2 and its homolog proteins (HSTs) in yeast, Sir2 is the only protein exclusively localized in nuclei, whose activity is critical for both gene silencing and extension of yeast life-span (reviewed in Guarente, 2000). Based on protein sequence homology analysis, mouse Sir2α and its human ortholog SIRT1 (or human Sir2α) are the closest homologs to yeast Sir2 (Imai et al., 2000; Frye, 1999, 2000). However, their biological functions remain unclear.